1. Field of the Invention
This invention relates to a transmission unit, particularly to a transmission unit which acts in the manner of a differential between driven axles of a motor vehicle. The transmission unit comprises an epicyclic gear train having a rotatable input member, a second rotatable member, and a rotatable output member, and the transmission also comprises a liquid friction coupling which is connected to said second rotatable member.
2. Description of the Prior Art
British Patent Specification No. 1,357,106 discloses a transmission unit which is of that type and in which the epicyclic gear train consists of a planetary gear train having a planet carrier which is driven by the engine whereas the sun gear and the internal gear are connected to respective axles of the vehicle and to respective sections of the liquid friction coupling. In that transmission unit the epicyclic gear train constitutes a differential between driven axles of a vehicle and the liquid friction coupling serves to lock the differential. The same printed publication discloses also the use of a liquid friction coupling as an interaxle differential. Both embodiments have various disadvantages, which will now be explained.
When the vehicle is cornering so that the wheels of the two axles rotate at different speeds, the resulting speed difference between the sections of the liquid friction coupling gives rise to a cunstraining torque, which opposes the steering of the vehicle. The steering will become particularly difficult at low temperatures because the constraining torque will then be increased by the high viscosity of the liquid in the liquid friction coupling. Besides, in the two known embodiments the two axles cannot be uncoupled from each other although such uncoupling will be necessary, e.g., when the vehicle is being hauled in a condition in which one axle is raised and its wheels are blocked, or when the vehicle is to be tested on a test stand that is designed to take up only one axle. An uncoupling of the two driven axles from each other must also be possible, as a rule, in a vehicle having a brake system provided with an antiblock device. Because both sections of the liquid friction coupling are rotated, a considerable structural expenditure will be involved in an arrangement in which the coupling can be externally controlled to modulate the relationship between the speed difference between the sections of the coupling and the torque limit of the transmission unit.